It is well documented that lying during interviews takes up more cognitive energy than telling the truth. A new study by the University of Portsmouth found that investigators who used this finding to their advantage by asking a suspect to carry out an additional, secondary, task while being questioned were more likely to expose lie tellers. The extra brain power needed to concentrate on a secondary task (other than lying) was particularly challenging for lie tellers.
[...] "Our research has shown that truths and lies can sound equally plausible as long as lie tellers are given a good opportunity to think what to say. When the opportunity to think becomes less, truths often sound more plausible than lies. Lies sounded less plausible than truths in our experiment, particularly when the interviewees also had to carry out a secondary task and were told that this task was important."
[...] Professor Vrij said: "The pattern of results suggests that the introduction of secondary tasks in an interview could facilitate lie detection but such tasks need to be introduced carefully. It seems that a secondary task will only be effective if lie tellers do not neglect it. This can be achieved by either telling interviewees that the secondary task is important, as demonstrated in this experiment, or by introducing a secondary task that cannot be neglected (such as gripping an object, holding an object into the air, or driving a car simulator). Secondary tasks that do not fulfil these criteria are unlikely to facilitate lie detection."
So if you think your significant other is hiding something from you, grill them when they're driving a car.
Aldert Vrij et al., The Effects of a Secondary Task on True and False Opinion Statements [open], Int J Psychol Behav Anal, 8, 2022